JP-A-9-10238 discloses a dental irradiation unit in which an array of light-emitting diodes is arranged on he hemispherical surface of an optical conductor member, in the shape of a conical sector, made from quartz or plastic, the tip of which merges into a rigid optical fiber rod. The beams of the light-emitting diodes are focused by the optical conductor member by reflection at the conical wall and passed into the rigid optical fiber rod.
WO-97/36552 discloses a further dental irradiation unit in which a planar array of light-emitting diodes with parallel optical axes is situated opposite the curved entrance surface of a likewise conical optical conductor member. This condenser is coupled on the output side to an optical conductor and filled, if appropriate, with an optically transparent liquid.
Apart from the fact that such conical optical conductor members such as have been described in the above publications, or else in WO 99/16136 (FIG. 4), are expensive to produce, and increase the weight of the unit, they also cause substantial radiation losses. These result from the fact that with each reflection of a light beam at the conical wall of the optical conductor member the beam is deflected by double the cone angle from the optical axis. This leads even after a few reflections to the fact that the angle for total reflection in the optical conductor is exceeded and the beams exit from the optical conductor or, in the case of silvering of the optical conductor, the beams even are reversed in direction and are therefore guided back not to the light exit orifice but to the light entrance orifice. Such arrangements therefore function only for a fraction of the light irradiated by the LEDs whose beams are tilted relative to the optical axis only in a very narrow angular range. Consequently, the predominant portion of the light emitted by the LEDs cannot be used to illuminate the treatment surface, since the luminous cones with the aid of which LEDs emit light usually have usually have substantially larger aperture angles.
Moreover, WO-99/16136 (FIG. 6) discloses a unit with a multiply conical optical conductor member in the case of which a plurality of annular light entrance surfaces are placed in front of a circular light entrance surface. In this case, the multiply conical optical conductor member directs the light from the first circular light entrance surface into the central region between the first annular light entrance surface. Together with the light from the LEDs, which irradiate this light entrance orifice in annularly arranged fashion, it is now conducted into the center of a further combination of an LED ring and annular entrance surface. The light thus collected is now guided to an exit orifice by the optical conductor, which is once again conical in its further course.
For the reasons named above, with this arrangement as well only a small fraction of the beams emitted by the LED reaches the light exit orifice and thus the site to be irradiated. By connecting a plurality of conical optical conductor members in series, the efficiency of the regions situated furthest removed from the exit orifice is reduced once more even by comparison with a singly conical condenser. In addition, the fabrication of the multiply conical optical conductor is, once again, more complicated and more expensive.
There are also other designs of irradiation units based on LED that manage without a conical optical conductor member and the disadvantages associated therewith. Thus, optical positive lenses for concentrating the beams emitted by an LED array and focusing the latter onto the light entrance surface of an optical conductor are proposed in JP 08-141001 (FIG. 1) and WO 99/35995 (FIG. 4). In this case, the totality of the beams emitted by the individual LEDs in the direction of the positive lens in deflected. The deflection in the desired direction and focusing of the beams succeed, however, in turn, only for the fraction of the beams that strike the positive lens in a substantially parallel fashion or, depending on the size of the light entrance surface, deviate slightly from the direction. A substantial fraction of the beams cannot be guided by the positive lens onto the light entrance orifice, and is therefore lost for the irradiation of the treatment site.
In the case of the arrangement shown in FIG. 1 of WO 99/35995, 9 LEDs are aligned individually in the direction of an optical conductor, in which case it was possible to observe only a partial hardening of a light-hardening sample. This is to be ascribed to inadequate light power as a consequence of non-optimum launching of light, as well as of the small number of the LEDs that can be used with the arrangement described.
Another unit described in WO/00/13608 is based on a similar arrangement of the LEDs in relation to the light entrance orifice of an optical conductor. The light power is increased by, on the one hand, making use of a conical optical conductor and, on the other hand, applying an increased working current (multiple times the nominal working current) to the diodes. The conical optical conductor also has the problems already described above. A further disadvantage caused by the high operating currents is that disproportionately high heat occurs at the LEDs, as a result of which the unit becomes hot after a short time, and cannot be used for a prolonged period of time until it has cooled down. In addition, the service life of LEDs suffers under the high operating currents, resulting in a continuous drop in light power over time.
The arrangements for LEDs in the form of individual semiconductor chips on a common substrate, as proposed in WO 99/35995 or EP-A-0 879 582 are also not without their problems. The individual elements heat one another up, resulting in setting limits for the light intensity and/or the service life. In addition, the production of such arrangements is substantially more complicated and expensive, since it is not possible to resort to standard components which can be handled effectively in mechanical terms.
It is therefore common to the above units that the achievable light powers are limited by the arrangements described there for the LEDs.